SUMMARY OF WORK Epidemiological observations reveal a high prevalence of elevated arterial blood pressure during colder months, however the mechanisms of this seasonal hypertension was not adressed. The objective of this project is to develop an experimental animal model of cold-induced hypertension and to investigate the mechanisms responsible for elevation of blood pressure during cold acclimation. Adult and aged Wistar rats acclimated to thermoneutrality (26_C) were exposed to cold (6_C) for 9 weeks followed by 5 weeks of rewarming. In adult rats the elevation of systolic blood pressure started two weeks after beginning of cold exposure and was 30 mmHg above the control level after six weeks of exposure. The elevation of blood pressure was preceded by a plasma volume expansion and was accompanied by an increased water consumption and elevation of whole blood viscosity. During the five weeks of rewarming the elevated blood pressure and blood viscosity did not return to normal, but plasma volume and water consumption normalized. Cold exposure of aged rats did not result in elevation of the systolic blood pressure, blood viscosity or plasma volume expansion, however, before cold exposure these parameters were already higher in aged than in adult rats. We believe that cold-induced elevation of blood pressure in rats represents the first naturalistic experimental model of volume-associated hypertension that will facilitate the development of treatment and prevention of this debilitating condition.